Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method and an apparatus for a cell to discover another cell.
Related Art
Long-Term Evolution (LTE) based on 3rd Generation Partnership Project (3GPP) Technical Specification (TS) Release 8 is a leading next-generation mobile communication standard.
As set forth in 3GPP TS 36.211 V8.7.0(2009-05) “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8),” physical channels in LTE may be classified into downlink channels, such as physical downlink shared channel (PDSCH) and physical downlink control channel (PDCCH), and uplink channels, such as physical uplink shared channel (PUSCH) and physical uplink control channel (PUCCH). The PUCCH is an uplink control channel used for transmitting uplink control information, such as a hybrid automatic repeat request (HARQ) acknowledgement (ACK)/non-acknowledgement (NACK) signal, a channel quality indicator (CQI) and a scheduling request (SR).
An evolution of 3GPP LTE, LTE-Advanced (LTE-A), is developing. Among various Release-12 technology work items, standardization of techniques for a small base station (small eNB or small cell) used to cover a smaller area than that covered by an existing macro-base station (or macrocell) is in progress. Base stations may be classified into macro-, pico- and femto-base stations depending on the size of coverage. A macro-base station is a generally used base station, which may cover a wider area than a pico-base station. Thus, the macro-base station may use relatively greater power than the pico-base station. The pico-base station may cover a small area for a hotspot or coverage hole. Generally, the pico-base station may use relatively lower power than the macro-base station. Thus, the pico-base station may have inferior connection reliability to the macro-base station. For a situation where a macro-base station and a small base station coexist, various methods may be used to efficiently utilize network resources. For example, traffic may be offloaded from the macro-base station to a small base station depending on a load condition of the macro-base station. When this method is used, the network load of the macro-base station may be adjusted to increase network efficiency. Further, by using difference in connection conditions between the macro-base station and the small base station, the respective base stations may process different kinds of quality of service (QoS) traffic. A UE may perform dual connectivity to connect to the macro-base station and the small base station at the same time so as to transmit and receive traffic.